1. Field of the Invention
The present invention relates to a laser diode (LD) array and more particularly, to an LD array including a plurality of LDs that have different oscillation wavelengths and that are arranged to form an array on a semiconductor substrate, which is applicable to the Wavelength-Division-Multiplexed (WDM) optical transmission systems, and a fabrication method of the LD array.
2. Description of the Prior Art
In recent years, with the increasing need for transmission of the multimedia information, much attention has been becoming attracted to the WDM optical transmission technique because this technique makes it possible to increase the capacity of information transmission through an optical fiber.
In the WDM optical transmission systems, the high-gain wavelength region (i.e., 1530 nm to 1562 nm) of an Erbium-Doped optical Fiber Amplifier (EDEA) is typically used. To effectively utilize this wavelength region, a plurality of LDs having different oscillation wavelengths within this bandwidth of approximately 30 nm need to be provided on a semiconductor substrate, producing an LD array.
A fabrication method of this conventional LD array is disclosed in a paper written by H. Yamazaki et al., First Optoelectronics and Communications Conference (OECC '96), Technical Digest, July 1996. In this method, a plurality of diffraction gratings having different pitches or periods for the LDs are formed on a semiconductor substrate by Electron-Beam (EB) exposure and etching processes. Distributed-FeedBack (DFB)-type LDs are formed on these gratings, respectively. Thus, the same diffraction gratings as those on the substrate are formed on the substrate-side surfaces of the guiding layers of these LDs.
Generally, the oscillation wavelength of a DFB-type LD is determined by the period of a grating of its semiconductor guiding layer. Therefore, the DFB-type LDs on the substrate will have different oscillation wavelengths corresponding to the period difference of the gratings.
Thus, the conventional fabrication method disclosed in the technical digest of OECC '96 has an advantage that the unit cost of the LDs is decreased compared with the case where a plurality of semiconductor substrates or wafers are used for obtaining the LDs with the different oscillation wavelengths.
However, the conventional fabrication method disclosed in the technical digest of OECC '96 has the following disadvantage.
Generally, the threshold current of a DFE-type LD is determined by the relationship between the oscillation wavelength .lambda..sub.dfb and the gain peak wavelength .lambda..sub.g. For example, when the difference of the oscillation wavelength .lambda..sub.dfb from the gain peak wavelength .lambda..sub.g, i.e., (.lambda..sub.dfb -.lambda..sub.g), which is termed the "detuning", becomes large, the threshold current of the LD becomes higher, resulting in difficulty in laser oscillation.
Also, when the directly-modulated DFB-type LD is used for optical communication or transmission systems, the time-dependent fluctuation of the oscillation wavelength .lambda..sub.dfb, which is termed the "wavelength chirping", needs to be small. The reason is that the large wavelength chirping will cause unacceptable waveform deformation or degradation after the long-distance transmission, which is due to the wavelength dispersion of optical fibers.
It has been known that the differential gain of the LD should be as large as possible for the purpose of suppressing the wavelength chirping, and that the oscillation wavelength .lambda..sub.dfb should be set at the short-wavelength side of the gain peak wavelength .lambda..sub.g to make the detuning negative. However, if the detuning (.lambda..sub.dfb -.lambda..sub.g) is excessively negative, the threshold current will become higher, as explained above.
As a result, it has been said that the detuning should be negative and have an absolute value of 20 nm or less in order to realize an LD characteristic with a low threshold current and low wavelength chirping.
From this point of view, the conventional fabrication method of an LD array disclosed in the technical digest of OECC '96 has a problem that all of the LDs on the substrate cannot be set to have an optimum negative detuning of -20 nm or less if the oscillation wavelength span of these LDs is over 20 nm. This is because the oscillation wavelength .lambda..sub.dfb of each LD is realized by changing the pitch or period of the diffraction gratings while all the LDs have the same gain peak wavelength .lambda..sub.g.
For example, in the LD array fabricated by the above conventional fabrication method disclosed in the technical digest of OECC '96, the LDs on the substrate have the different oscillation wavelengths .lambda..sub.dfb ranging from 1520 nm to 1563 nm and the same gain peak wavelength .lambda..sub.g of 1520 nm. Therefore, no laser oscillation occurs in the wavelength region longer than 1540 nm where the negative detuning is excessively large.
There is a similar problem in an LD array having DFB-type LDs and optical modulators of the electric-field absorption type, which are integrated on a semiconductor substrate.
The LD array of this sort has been expected as a compact light source for the WDM optical communication or transmission systems because it has a low wavelength chirping. The modulation characteristic of each optical modulator is dependent upon the difference between the absorption edge wavelength .lambda..sub.m of a corresponding one of the modulators and the oscillation wavelength .lambda..sub.dfb of a corresponding one of the LDs.
When this wavelength difference (.lambda..sub.dfb -.lambda..sub.m), which is also termed the "detuning", is smaller than a specific desired value, the optical absorption in the modulators is excessively large, resulting in an increased extinction ratio and a decreased optical output. Contrarily, when this wavelength difference (.lambda..sub.dfb -.lambda..sub.m) is larger than the specific desired value, the optical absorption in the modulators is excessively small, resulting in a decreased extinction ratio and an increased optical output.
Accordingly, from the view point of the proper optical output and the proper extinction ratio, this wavelength difference or "detuning", (.lambda..sub.dfb -.lambda..sub.m), needs to be set within the range of .+-.15 nm from the desired value in order to suppress the characteristic of the LD array within an acceptable fluctuation range. In other words, the wavelength difference (.lambda..sub.dfb -.lambda..sub.m) should be set within an optimum range of .+-.15 nm from the desired value.
As a result, the change of the oscillation wavelengths .lambda..sub.dfb of the LDs will cause some deviation of the wavelength difference (.lambda..sub.dfb -.lambda..sub.m) from the above optimum range in the optical modulators, which degrades the modulation characteristics of the modulators.